Figure 3.

The role of PP2A in functioning, recycling and transcytosis of PINs. Although in general, PIN1 and PIN2 are working in different cells, here we show them in a single cell compartment to show their differential transport and functioning. For simplicity, only vesicle traffic pathways directly or indirectly dependent on PP2A are shown. For example, other GEFs than GNOM and Rab GTPases involved in the traffic of PINs are not shown here. In root epidermal cells, the site of auxin efflux mediated by PIN2 is opposite to efflux mediated by PIN1 in the stele. Pathways of PIN1 and PIN2 polarity shift are partially interfering. GNOM is presumably participating in the transcytosis of PIN1 and PIN2 as well, but for PIN2, this happens mainly in root cortex cells where it directs auxin transport towards the root apex. GNOM pathway-mediated transport of PIN2 towards basal membrane for recycling purposes is probable (dashed purple arrows) but needs further evidence. Both PIN-mediated auxin efflux and their vesicular transport are dependent on protein phosphorylation. PINOID kinase (PID) phosphorylates both PIN1 and PIN2 at the apical membrane, while D6PK kinase phosphorylates PIN1 in the basal pole of a root cell. PP2A dephosphorylates PIN1 and presumably PIN2 at the apical pole and PIN1 at the basal pole. Question marks show yet not fully investigated events. At the basal membrane, PIN2 might be phosphorylated by a yet unknown kinase and if it remains in a phosphorylated state here, its recycling will be impeded. → (Red arrow) GNOM dependent transcytosis of PIN1 in the apical-basal direction. → (Pink arrow) Basal–apical shift of PIN1 is independent of GNOM. → (Purple arrow) The pathway of intracellular PIN2 traffic. Thick arrows highlight the transcytosis and recycling pathway of PIN1. AM—apical membrane; BM—basal membrane; LV—lytic vacuole; PVC—prevacuolar compartment (multivesicular body/late endosome); SNX1—SORTING NEXIN1 containing vesicle; the figure was constructed with the aid of BioRender.com.